1. Field of the Invention
This invention relates to a shock absorber and cushion for use between a flexographic printing plate and a printing cylinder during printing that compensates for variations in thickness, height and centricity of the materials and equipment used for printing to enhance the image quality and efficiency of the flexographic printing process without increasing printing pressure.
2. Description of Prior Art
Flexography is a printing process used primarily in the packaging and newspaper industries. The flexographic process requires that a raised surface plate be used to transfer ink onto a given substrate. This is unlike lithography, which works on a flat plane based on the principle that oil and water do not mix. The gravure printing process is a recess process in which cells are engraved into the print cylinder that are then filled with ink and then transferred to the substrate.
The flexographic printing process' unique capabilities include changing cylinder dimension (circumference) to accommodate As in most manufacturing and machine processes, there is a plus or minus tolerance in gauge (thickness) uniformity which may include: the print cylinder uniformity, both across and around the web; a tolerance in the material surface being printing on; and, the tolerance of the back cylinder which the substrate rides on as it maneuvers through the press in addition to other mechanical elements. Variations in tolerances require excessive pressure during printing on the flexographic plate to overcome inaccuracies which may smear and distort the print image such as halos and oval dots.
Currently the raised imaged carriers (flexographic plates) adhere to a print cylinder using various methods which include clamps, pins, vacuum and most commonly, an adhesive tape applied to a flat seamless cylinder. There are various types of adhesive tapes used to adhere the flexographic printing plate to the cylinder. Although there are many variations of adhesive tape materials available, the materials used are routinely lumped into the following three categories:
(1) Hard Tape--no significant or claimed cushioning affect. This tape is best used when large amounts of ink need to be applied at 100% strength (full strength). However, since this tape has no inherent ability to even out the mechanical tolerances of the printing press, more than minimal pressure is normally required. This pressure creates a distorted printed image appearing in various forms which may include hard edges around the outer portion of the line copy while leaving a halo adjacent to this hard edge. Depending upon impression required, text may be squeezed to a point where it begins to slur (elongated the print in a through-press direction). PA0 (2) Soft Tape--used as a cushion to allow even impression across and around the cylinder. This is because soft foam tape collapses or compresses under pressure in the areas that come into impression first which represent the largest circumference of the print package and must be impressed several thousands more until the entire image appears to be printing evenly and uniformly. Because of its softness, this material is used primarily when fine details or extremely small images are printed to help minimize the distortion that occurs under pressure with hard tape. Soft tape is traditionally used when printing half tones for screened pictorials, gradations and screen tints. Due to the soft nature of this cushioning element, the amount of pressure required to transfer a solid image is significantly compromised. PA0 (3) Medium tape used as a cushioning element considered to be of medium density. Medium tape is a compromise between the attributes of a soft tape used for printing fine graphics, and, hard tape used for images which need to print robust solids on the same printing surface using the same cushioning material.
The present invention eliminates or minimizes the negative attributes of the tape product(s) described above available today. This includes inconsistency in gauge of the raw material currently available which is said to vary by plus or minus several thousands of an inch. With foam technology, foam cells or voids are filled with air, and during impression, under high spots, air is forced away and needs time to return to cells and thus return to initial tape height or dimension prior to the next revolution of the press. Cell inflation delay requires the press to run at lower speeds when working with a softer foam tape, soft or medium. The slower drum speeds provide the time for the foam cell tape material to rebound between successive impressions. Throughout a very long print run, the adhesive tape material gradually loses ability to rebound. Constant monitoring is required throughout the run and most often results in color shifts and unacceptable print at some point in time, which is normally over one million impressions--but in most cases not greater than three million impressions.
U.S. Pat. No. 3,285,799 discloses a printing blanket for long periods of use in offset lithography which is composed of a polymeric film and woven backing, an ink transfer layer, and a resilient compressible support layer. The support layer has an external surface subdivided by grooves which leaves flat surfaced islands. The blanket is used as an intermediate to transfer an ink image from a printing plate to paper. The support layer has a durometer of at least 60 Shore A. The support layer contains at least about 0.005 cubic inches of voids per square inch of blanket surface but total void volume does not exceed 40%.
U.S. Pat. No. 5,325,776 discloses a cushioning backing sheet metal material positioned between a flexographic printing cylinder and a flexible printing plate. The cushioning sheet is an elastomeric material containing widely spaced, closed cell voids which provide pockets within which the encapsulated air can be pneumatically compressed when force is applied, and which all rebound rapidly when the force is relieved. A disadvantage of the closed-cell cushioning material fatigues and looses compression and resilience qualities, and thus print quality deteriorates.